Problems Saving Images and Sizes

[macease]

Hello. I have some jpeg images, and am having some problems with changing the sized. When I change their size and then save them, when I reopen them the size has changed since I saved it. And I don't see a clear pattern. Sometimes they revert to a previous size and sometimes to something else completely. Also, as you can see in the screenshots, sometimes when I adjust the size, even with the same dimensions the file size is different. What's going on here? And how do I fix it?

 

[Slartibart]

Which version of MacOS? Which software do you use?
What means the file size differs? Does the pixel count change? Or are you referring to changes in file size each time you just open a picture?

 

[macease]

Thanks for your help. I'm using Macbook air Mac OS 11.4, and I'm using Preview to open the jpeg images. When I change the width or height dimension of the image and save it, and then open the file again, the file size has gotten smaller since I saved it. See the screenshots: Screenshot 1 is when I changed an image height from 2500 to 2600. Then I clicked "ok" and saved it. Then I quit Preview. Then opened the image again. Screenshot 2 is when I open the image file again and check the size of the image.